I'm confused about the correct case to use in a temporal complement: do you need the accusative?
I'm asking about the expression of a well determined time (that does not continue).
Examples may be:
...

"Dieses Jahr", "diese Woche", "diesen Sommer" are all answers to the question "When?".
According to the rules one should use dative to answer (for example "diesER Woche"), however accusative is used. ...

I often hear "dann treffen wir uns gegen 8 Uhr irgendwo". How precise is this? Is it ±5 min or ±10 min or does it depend on the situation?
Are there other words which means "approximately at eight", ...